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When we set up OutputSurface as overlay, it will always have z-order of
0, since z-order 0 is defined as primary plane.

R=dcastagna@chromium.org

Bug: 963597
Change-Id: I6b399b13ba4d447686b8d957dd50d3ca6011c912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695988Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676153}